The invention relates to a preform for producing a structural unit made of fiber reinforced material. The preform is insertable in a form tool used in an RTM process in which the preform is resin-impregranted and cured. The form tool comprises a metal form with an upper mold and a lower mold, with the metal form enclosing the preform.
The RTM (resin transfer molding) process is used to produce a structural unit made of fiber reinforced material. The term RTM process includes a multitude of different processes. In the RTM process, it is known to use a sealing form tool which completely encloses a preform. The shape of the preform is the same as that of the structural unit. Thick-walled monolithic structural units which require precise external dimensions are made according to an RTM process which uses a metal form. The metal form comprises an upper mold and a lower mold which can be joined to form a seal. The upper mold and the lower mold as well as the insert components enclose the structural unit and result in a closed and sealed form tool. The air in the form tool can be evacuated via a connector. The connector is connected to a suction device and an injection device. Both devices are controllable. The preform, which corresponds to the structure of the structural unit, is inserted in this form tool and subsequently impregnated with a reaction resin mass before being cured. The reaction resin mass is a mixture of resin and hardener, for the sake of brevity subsequently referred to as xe2x80x9cresinxe2x80x9d. The result is a structural unit made of fiber reinforced material.
The preform comprises reinforcing fiber material or layers of fiber materials which can already be spatially arranged. They can also already be fabricated so as to comprise other structural sub-units.
The metal form as a form tool comprises a base plate, a so-called bell, and forming or molding insert components which can be inserted between the base plate and the bell. The insert components determine the external shape of the structural unit. Between the base plate and the bell, at least on the circumference, a sealing means is arranged so as to prevent the resin from escaping during injection. This metal form can be clamped between an upper and a lower heatable press plate. In addition, the metal form is necessary because injection of the liquid resin takes place at high pressure (up to 30 bar) and high temperature (approx. 180xc2x0 C.).
To ensure homogenous impregnation of the preform with resin, the so-called insert components must be made so as to be dimensionally accurate in relation to each other and in relation to the bell and the base plate, so that no flow channel can arise between the resin inlet on one side of the form tool and the outlet on the opposite side of the form tool. Formation of such a flow channel is frequently caused by a lack of dimensional accuracy in the insert components or by unevenness on the fitting surface between the base plate and the bell. Such lack of dimensional accuracy can also cause leakages of vacuum and of resin from the closed form tool in spite of the sealing means, causing uncontrolled resin discharge. This jeopardizes continuous and homogenous impregnation of the preform.
In view of the above, it is an object of the invention, during production of a structural unit of fiber reinforced material according to the RTM process, to prevent resin discharge from the form tool enclosing the structural unit during resin impregnation of the preform. The invention further aims to avoid or overcome the disadvantages of the prior art, and to achieve additional advantages, as apparent from the present specification.
The above objects have been achieved according to the invention, in a preform unit for producing a structural unit of fiber reinforced material, in a forming system and in a forming method.
The preform unit according to the invention comprises a preform body and an envelope in which the preform body is enclosed. For simplicity in this specification, the preform body itself or the overall preform unit are sometimes referred to merely as xe2x80x9cthe preformxe2x80x9d.
The preform body of the preform unit comprises reinforcing fiber material or layers of fiber material which can already be spatially arranged and fabricated so as to comprise other structural sub-units. The term xe2x80x9creinforcing fibersxe2x80x9d is broad and general, and refers to any fibers, strands, threads, whiskers or the like having a generally elongated shape, any suitable length, and being arranged as a woven web, a knitted sheet, a non-woven fleece, an oriented fiber mat or batt, or a random non-oriented fiber mat or batt, for example.
The envelope of the preform unit comprises an envelope sleeve or wall and at least one connector or closure arrangement (generally at least two) with closeable apertures. The envelope completely encloses the preform body. One of the two closeable apertures of the envelope is required for evacuating (by suction) air from the envelope so that the envelope hugs the preform with a positive fit. The preform with the envelope surrounding it is inserted in the form tool after the air has been evacuated from the space between the envelope and the preform. The other of the two closeable apertures is for injecting resin into the envelope and into the preform therein after the envelope has been evacuated and placed in the form tool. The envelope preferably comprises an elastic plastic material enclosing the preform, e.g. shaped as a hose, tube or pillow. Other shapes are also possible. The plastic material is air-impermeable and resistant to chemical attack by the resin. The envelope can either enclose or not enclose the forming insert components, as desired.
By using an envelope to enclose the preform, it is no longer necessary to provide a seal for the form tool. This results advantageously in a considerably simplified design of the form tool. Moreover, by using an envelope, the previous use of release agents between the insert components and the form tool or between the preform and the form tool is now no longer necessary. The envelope ensures that the form tool does not come into contact with resin. Thus, further use of the release agent is no longer necessary. Since the form tool no longer comes into contact with resin, there is a significant reduction in expenditure for subsequent cleaning of the form tool. The previously necessary use of solvents for removing resin from the form tool is no longer required.
The envelope also provides a convenient packaging for the preform. Through the use of the envelope, the preform is ready to be stored and transported.